1. Field of the Invention
The present invention relates to fuel nozzles, and more particularly to air-blast type fuel nozzles for gas turbine engines.
2. Description of the Related Art
Since the efficiency of a gas turbine engine increases as the pressure ratio increases, it is desirable to operate at as high a pressure ratio as feasible. State-of-the-art gas turbine engines operate at pressure ratios much higher than gas turbine engines of the past. One of the factors limiting the pressure ratio at which gas turbines operate are the physical properties of the materials which are used in making the engine components. Recent advances in materials science and engineering have provided materials for compressor and turbine blades and housings which can withstand the extremely high temperatures and pressures commensurate with the high pressure ratios at which state-of-the-art gas turbines operate.
The fuel flow rate to a gas turbine engine increases as the pressure ratio at which the engine operates increases. In like manner, the range of fuel flow rates from starting to full load power of the engine increases as the pressure ratio at which the engine operates increases. The pressure drop across a fuel nozzle for a gas turbine engine is proportional to the square of the fuel flow rate through the nozzle. Hence, at the higher fuel flow rates required for high pressure ratio engines, the pressure drop across the fuel nozzle becomes exceptionally high, for some engines as high as 2,500-3,000 p.s.i., with resulting increases in the complexity and cost of manufacture of the fuel nozzles to withstand the high pressure differentials.
One method of limiting the pressure drop across the fuel nozzle in high pressure ratio gas turbine engines is to minimize the fuel flow through the nozzle during start-up of the engine, thus limiting the upper end of the fuel flow range. Gas turbine engines commonly employ air-blast type fuel nozzles which utilize the pressure differential across the fuel inlet port of the fuel nozzle to atomize the fuel. Thus, at low fuel flow rates, and corresponding low pressure differentials across the nozzle, air-blast type fuel nozzles exhibit a diminished capacity to provide uniform sprays having fine particle sizes. Furthermore, air-blast type fuel nozzles are normally constructed of three concentric passages wherein air flows through the innermost passage, fuel flows through the intermediate passage, and air flows through the outermost passage. The separate air and fuel flows are mixed upon exiting from the nozzle. This three concentric passage configuration results in increased cost and complexity of manufacture of the nozzles.
It is an object of the present invention to provide a fuel nozzle for a gas turbine engine which provides fine droplet sizes in the fuel spray exiting therefrom at relatively low fuel flow rates and corresponding low pressure drops across the nozzle.
It is a further object of the invention to provide a method which utilizes kinetic energy in an air flow stream in the nozzle in combination with the pressure drop across the nozzle to break up fuel exiting from the nozzle into fine droplet sizes, at relatively low fuel flow rates.
It is a further object of the invention to provide a system for controlling the fuel spray angle exiting from a fuel nozzle by regulating the air flow rates through selected passages of the nozzle.
It is a further object of the invention to provide an air-blast type fuel nozzle for a gas turbine engine which is less complex and less costly to manufacture than prior art air blast type fuel nozzles.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.